To extract gold from the mineral, especially the fine grained and submicron gold in the pyrite, arsenopyrite ore and other sulfide ores, it must first completely remove arsenic in these minerals so as to effectively extract gold. It has been a big technical barrier in the gold production about how to completely remove the arsenic in the gold concentrate and also not to cause environment pollution, which has been restricting the gold output.
Conventional dearsenization method is to conduct oxidizing roasting of the arsenic gold ore concentrate or gold bearing arsenopyrite ore to oxidize the arsenic in the mineral into volatilized As2O3 and arrive at the purpose of dearsenization. Although this method is simple, it has four disadvantages: (1) In the roasting process, since some arsenic and gold generate a kind of low boiling point and volatilized arsenic and gold compound under the roasting temperature, about 900° C., thus greatly lowering the gold recovery. The gold loss percentage is often up to 25-35%. (2) It is hard to get qualified arsenic product. Since the Sb, Bi, Pb, Hg, Zn and other contaminating metals also volatilize and mix with As2O3 by form of oxides, As2O3 cannot meet the product requirement on purity and cannot be sold. To utilize the As2O3 byproduct, conventional method is to put As2O3 into normal pressure electric heating vertical retort reducing furnace and use carbon to reduce As2O3 into element arsenic, involving heavy labor intensity. (3) Both oxidizing roasting process and the process reducing As2O3 into element As must deal with virulent As2O3. Each operation link is hard to avoid environment pollution by As2O3, especially even hard to assure the personal safety of operators. (4) It is not possible to remove arsenic. Since it is difficult to control the temperature of oxidizing roasting, some arsenic is left in the slag by form of oxide. Some arsenic turns into iron arsenate under high temperature and is left in the slag. The higher the temperature is, the higher arsenic content in the slag, which is not favorable for gold extraction in the next step.
To overcome the above disadvantages of using the As2O3 as raw material to produce element arsenic in the normal pressure reducing furnace, some research units made some small tests to directly extract element arsenic from arsenic concentrate by means of the vacuum process, such as the kilogram level test to remove arsenic from the cobalt ore in the existing technologies (China Non-ferrous Metal Journal, Book 4 Issue 1, 1993), which aimed at creating conditions for the next-step wet method extraction of element cobalt. The test theory was to enable thermal decomposition of the Co, Fe, Ni and As compound in the cobalt concentrate under vacuum conditions and separate out element arsenic. The experimental conditions were: residual pressure 6-10 Pa and temperature 1100-1200° C. But the experimental result had many problems: (1) arsenic grade cannot meet the international requirement of 99% of arsenic and can only reach 76-92% of crude arsenic. Even the further distillation was also hard to reach the product requirement and involved high cost. (2) Since the smelting temperature was up to 1100-1200° C. and materials were under semi-molten state, it was difficult to discharge slag for application in industrial production. (3) The exhaust issue has not been solved. When arsenic vapor and vapor were generated in the furnace, they would cause the splash of molten materials and produce large quantity of dust polluting the arsenic product and hard to get qualified arsenic (4) Arsenic content in the slag was up to 10-18%, which brought not only low arsenic recovery, but also the problem of further dearsenization requirement in the subsequent smelting sequence. Another example is some medium and small tests made by means of existing horizontal type horizontal type rotary vacuum furnace to extract element arsenic from arsenopyrite ore, which still has many problems and has not been used for industrial product till-now. Main problems are as follows: (1) The arsenic corrosion problem of rotary furnace has not been solved, leading to low furnace life and being not suitable for industrial production. (2) The furnace rotation generates large quantity of dust in the process of continuous stirring of materials, which seriously pollutes the product and is its second fatal weakness. (3) The exhaust problem has not been solved. Under high temperature, vapor generated from crystal water in materials directly enters the vacuum unit, often enables the impossible normal operation of vacuum pump and also leads to failure of vacuum solenoid valve. The requirement on vacuum degree cannot be guaranteed. Sometimes, the water accumulated in the vacuum pump leads to the oxidization of pump parts and rejection of vacuum pump. These accidents happened often lead to leakage of vacuum system and As2O3 pollution. (4) Due to continuous rotation of furnace shell, it is very difficult to measure the actual temperature in the rotary body. Plus, such furnace type integrates smelting chamber and crystallization chamber in the same furnace shell. It is more difficult to control temperature at connection between both chambers. (5) Deslagging and product stripping cannot be conducted at the same time. It must first conduct product stripping and then deslagging, which greatly extends the operating time. (6) Since the effective charging size of smelting chamber (material chamber) of horizontal type rotary furnace is small, and must be less than half the actual size of smelting chamber, otherwise, the materials will flow out of the vent hole (i.e. charging hole) upon rotation and continuously flow into the crystallization chamber and mix with the product. The above problems lead to the fact that the horizontal type rotary vacuum furnace cannot be used for industrial production.
Another example is 100 g small tests made by given arsenopyrite ore under vacuum conditions through thermal decomposition and extraction of element arsenic. The test ore charge is pure arsenopyrite ore. Firstly, the mineral is subject to cleaning to remove most impurities, and subject to leaching with the Iron(III) sulfate to remove FeS2 and other sulfides and get pure arsenopyrite ore as charging material. Although the qualified element arsenic can be got, it is easy to realize for small tests using pure arsenopyrite ore as charging material, and industrial production cannot meet such strict conditions. And the 100 g level vacuum furnace has its integrated smelting chamber, crystallization chamber and dust chamber. After furnace shutdown and temperature lowering, the particles of element arsenic are removed from the shell wall (crucible wall). Such tests can only show that the established fact of vacuum thermal decomposition and extraction of element arsenic.
Someone also made tests to adopt minor negative pressure operation in the furnace and enable thermal decomposition of arsenopyrite ore and extraction of element arsenic. So-called minor negative pressure is that the pressure difference between inside and outside the furnace is about 10 mm water columns. But the minor negative tests can also only show the established fact of thermal decomposition of arsenopyrite ore and extraction of element arsenic, and cannot eliminate the conditions of generating As2O3, far away from the industrial production.